Solar concentration systems typically entail optics (mirrors or lenses) to focus a large area of sunlight, or solar thermal energy, onto a small area. The solar thermal energy may drive a heat engine, such as a steam turbine, which may be further coupled to an electrical power generator to convert a portion of the solar thermal energy into electricity. Solar concentration systems may also drive a thermochemical reaction to generate a fuel that chemically stores a portion of the solar thermal energy. Water splitting, gasification of coal, and reforming of methane are all under investigation as potential solar thermochemical fuel production techniques. Solar concentration systems may drive other important reactions on an industrial scale as well, such as CO2 reduction into CO, for example.
Many solar thermochemical reactions entail a redox cycle. In a water splitting reaction to produce hydrogen from water, a metal-oxide redox pair is thermally reduced and the reduced reactive media then drives decomposition of water. The metal oxide is then reduced again to repeat the cycle. While identifying advantageous metal-oxides is currently a subject of research, thermodynamic considerations dictate the thermal reduction portion of the cycle generally requires a high temperature, typically between 1000-2000° C., depending on the reactive oxide chosen and other conditions in the system.
Solar thermochemical reactors can take many forms, affording more or less efficient fuel production, scalability, etc. One conventional system utilizes a honeycomb substrate that is coated with the reactive oxide. The honeycomb substrate is alternately exposed to collected solar energy to heat the system and reduce the reactive oxide, and to a reactant gas, such as H2O in the case of water splitting, to generate fuel. Such a reactor is essentially a fixed bed, and as such, suffers temperature non-uniformities and low thermal efficiency because much of the solar energy is expended on heating non-reactive portions of the bed (e.g., honeycomb substrate) and is ultimately rejected from the system as waste heat, rather than utilized for fuel production. Also, with each redox cycle, the entire system undergoes extreme thermal cycling, leading to component fatigue.
Two-step thermochemical cycles are a conceptually simple, theoretically highly efficient, and promising approach for solar fuel production. In the first step, thermal reduction, a reactive material (oxide) is partially or fully reduced at high temperature (typically 1600-1800 K). In the second step, fuel production, the reduced oxide is exposed to steam or CO2 at a lower temperature (1100-1500 K), to produce H2 or CO.
Efficiencies achieved in experiments have been much lower than theoretical limits, largely owing to the performance of existing reactive oxides. The low reversible oxygen capacity of these oxides results in low H2 or CO yields per mole oxide per cycle, leading to large energy requirements for oxide heating. Increasing the per-cycle yield has proven to be challenging, requiring high thermal reduction temperatures (TTR) and low thermal reduction pressures (pTR). High temperatures cause excessive aperture radiation losses and require the use of specialized construction materials, limiting TTR to <1800 K. Low pressures can be achieved by inert gas sweeping or vacuum pumping, both facing limitations for reasonably-scaled commercial systems (1-10 MW). Sweeping requires large amounts of high-purity gasses and extensive high-temperature heat recovery, whereas pumping requires excessively large vacuum pumps, limiting the minimum achievable pTR.
A system which avoids many of the difficulties and efficiency limitations associated with existing reactors would advance the art of solar thermochemical fuel production.